Organopolysiloxane emulsion compositions are often used in such products as cosmetics, personal care products, household products, and parting agents. The emulsion required in these applications is an emulsion of a high-viscosity organopolysiloxane having a small particle size and good age stability. However, when high-viscosity organopolysiloxane is directly emulsified, the size of emulsified particles is of the order of several microns at the lower limit, i.e., it is difficult to form smaller particles than this level. The resulting emulsion has poor age stability. For the purpose of obtaining emulsified particles having a small particle size and good age stability, studies were made on the method for preparing an emulsion via emulsion polymerization.
For example, it is known from Patent Documents 1 and 2: JP-B S34-2041 and JP-B S41-13995 that cyclic siloxane oligomers are emulsified before emulsion polymerization is conducted in the presence of strong acids or strong bases. With these techniques, emulsions having an emulsified particle size of up to 300 nm are obtainable.
Recently, octamethylcyclotetrasiloxane (D4) becomes a matter of concern as environmental load substance. Products having a reduced content of D4 are required. It is known that with the methods of Patent Documents 1 and 2, the organopolysiloxanes in the resulting emulsions have a D4 content as high as 40,000 ppm or more. Attempts were made to reduce the D4 content.
One known approach (Patent Document 3: JP 5382273) is a method involving the steps of emulsifying a silanol-terminated organopolysiloxane having a kinematic viscosity of 3,000 to 100,000 mm2/s at 25° C. and an octamethylcyclotetrasiloxane (D4) content of up to 1,000 ppm and effecting emulsion polymerization in the presence of an acid catalyst at a temperature of lower than 40° C. Allegedly, this method yields an emulsion of an organopolysiloxane containing 3,000 ppm or less of D4. However, since the monomer subject to polymerization has a high kinematic viscosity of 3,000 to 100,000 mm2/s, the method requires high mechanical shear to reduce the particle size in the emulsion of the monomer. As a result, sometimes the emulsion has poor age stability.
Another known approach (Patent Document 4: JP-A 2014-512418) is a method involving the steps of emulsifying a silanol-terminated organopolysiloxane having a kinematic viscosity of 2,000 to 150,000 mm2/s at 25° C., water and a surfactant and effecting emulsion polymerization in the presence of an acid catalyst at a temperature of up to 16° C. This method can control the extra formation of D4 in the resulting organopolysiloxane. However, since the monomer subject to polymerization has a high kinematic viscosity of 2,000 to 150,000 mm2/s, the method requires a large amount of surfactant in order to prepare an emulsion of submicron particles.
There is a need to establish a method of preparing an organopolysiloxane emulsion composition having a submicron particle size and good age stability, without a need for an excess of surfactant, while suppressing the extra formation of D4 in the organopolysiloxane.